In a cylinder head for an internal combustion engine, an overlay welded portion is formed by supplying a melted metal material to an inner circumferential groove portion formed on an inner side of a circumferential wall portion defining a port, and this overlay welded portion serves as a valve seat. For example, in Japanese Unexamined Patent Application Publication No. 2015-048789, the overlay welded portion is formed by laser cladding process.
Meanwhile, the cylinder head is provided with a thick portion on a part of the circumferential wall portion in some cases, in order to ensure the rigidity against the combustion pressure. It is desirable to set the position of such a thick portion near the center of a combustion chamber on which high combustion pressure tends to act.
In the process of forming the overlay welded portion on the inner circumferential groove portion of the circumferential wall portion as described above, the melted material solidifies. In the vicinity of the thick portion of the circumferential wall portion as described above, more heat is transferred from the melted material to the thick portion, and the melted material solidifies in a relatively short time. In contrast, in the vicinity of a thin portion, it is difficult to release the heat from the melted material, and the melted material solidifies in a long time. The difference in time in which such a melted material solidifies might increase the residual stress of the overlay welded portion in the vicinity of the thick portion.
Also, the inner temperature of the combustion chamber increases during operation of the internal combustion engine, and the inner side of the combustion chamber is cooled when the internal combustion engine stops. Accordingly, the overlay welded portion is heated and cooled, and this temperature difference causes the thermal stress on the overlay welded portion. Such thermal stress increases in the center side of the combustion chamber, which tends to be high in temperature by combustion.
Therefore, if the position of the thick portion is set in the vicinity of the center of the combustion chamber, thermal stress in addition to the residual stress act on the overlay welded portion, so that the local stress might concentrate on the overlay welded portion. The local stress concentrates on the overlay welded portion formed as a valve seat in this manner, which might influence the durability.